1. Field of the Invention
The present invention relates to a board for an inkjet printing head which performs printing operation by ejecting ink onto a printing surface of a printing medium, and to an inkjet printing head using the board. In particular, a board for a printing head, to which the present invention is applied desirably, is that in which elements to generate energy are arranged on one of its sides (top surface). The energy is used for ejecting ink in a direction perpendicular to the board in accordance with its drive. In addition, the board includes a plurality of ink supply ports each shaped like a long groove, which penetrate through the board from a side (back surface) opposite to the top surface, and which introduce the ink onto the aforementioned elements. The board also includes electrode pads along outer peripheral edges of the board, the edge being parallel to short sides of the ink supply ports each shaped like a long groove.
2. Description of the Related Art
FIGS. 9 and 10 show a conventional example of such an inkjet printing head (hereinafter also referred to as a “printing head”) used for an inkjet printing apparatus. FIG. 9 is a perspective view showing a part of the board by cutting away a part of a member in which ejection openings and the like are formed. FIG. 10 is a plan view mainly showing power supply wires on the board for the printing head.
In the case of the printing head of this type, as shown in FIG. 9, a plurality of heating portions (hereinafter also referred to as “heaters”) 802 are provided to the top surface of a board 805, and the heating portions are arranged in a staggered pattern in which an arrangement of heating portions along one side of an ink supply port 803 is shifted from an arrangement of heating portions along the other side of the ink supply port 803, the port penetrating through the board. In addition, a member in which ink ejection openings 801 and ink passages 804 are formed is disposed on the board 805. The ink ejection openings 801 and the ink passages 804 correspond to the plurality of heating portions 802, and the ink is ejected through each of the ink ejection openings 801 and the ink passages 804.
The following components are provided on the board 805 as shown in FIG. 10 for the purpose of ejecting the ink by selectively driving the plurality of heaters 802 in accordance with printing data. The components include:
common wires 902a and 902b which are connected to the power supply;
power supply wires 910 for supplying power to the heating portions 802 respectively;
driving elements such as transistors (located in hatched parts in the drawing, constituting as lower layers under the heating portions and their related wire layers, and formed in the board); and
wires or circuits, such as common wires 904a and 904b which are connected to the ground (GND).
In addition, the common power supply wires and the common GND wires can be electrically connected to the outside of the board through electrode pads 903 respectively. Incidentally, illustration of required interlayer insulating films and protective films is omitted from the drawing. The interlayer insulating films are arranged in conjunction with layers for forming the heaters, electrode layers and driving elements. The protective films are used for protecting the heaters, electrode layers and driving elements from the ink.
In the inkjet printing head having the above-described configuration, ink is held in a state in which the ink forms a meniscus in the vicinity of each ejection opening 801. The heating portions 802 are selectively driven in accordance with recording data in this state, and the thermal energy generated is utilized to sharply heat and boil the ink on a heat applying surface. Thus, ink can be ejected by the pressure of bubbles generated at this time.
Incidentally, electric energy or power which is applied to the heating portions in order to eject ink is one of important factors which influence the ejection. That is, when the applied electric energy varies, a foaming phenomenon also varies accordingly, and favorable ejection may not be performed.
In a case where, for example, the applied driving energy is small, the film boiling phenomenon of the ink is prone to become unstable due to a lack of energy. As a result, a satisfactory film boiling does not take place. Accordingly, this fluctuates ejection speed, ejection direction and an amount of ejected ink. In some cases, these may deteriorate the quality of printed images. In contrast, in a case where the applied driving energy is high, excessive thermal energy gives mechanical stress to the heating portions 802, and changes the film quality. These may also cause the ejection failure as described above. In the worst case, the printing head may be broken. With this taken into consideration, it is desirable that energy to be applied to each of the heaters should always be almost entirely uniform.
On the other hand, known factors that cause fluctuations in energy applied to each heating portion also include one caused by the fact that the number of heating portions simultaneously driven changes in one recording head. That is, if the number of heating portions simultaneously driven changes depending on recording data or the like, a voltage drop generated changes accordingly. As a result, the driving energy of each heating portion changes.
A configuration disclosed, for example, in Japanese Patent Application Laid-open No. 10-44416 (1998) has been heretofore one of countermeasures against these problems. In the case of this configuration, as shown in FIG. 10, each common wire between the corresponding heaters 802 and the corresponding electrode pad as well as each common wire between the corresponding driving elements and the corresponding electrode pad is divided into the plurality of sections. Each of the sections includes a unit consisting of a certain number of heating portions and driving elements. Further, values of wire resistances of the common wires 902a, 902b, 904a and 904b are made approximately equal to one another. This configuration makes it possible to decrease the difference in voltage drop between the case where all the heaters are driven for any one of the common wires corresponding respectively to the units and the case where one heater is driven for the same common wire. Furthermore, among of the heaters connected to each of the common wires, a single heater is to be driven at a time. This makes it possible to eliminate the difference between the case where all the heaters would otherwise be driven for the same common wire and the case where the single heater is driven for the same common wire. Accordingly, this makes it possible to always apply constant driving energy to each of the heaters.
This configuration is adopted for the purpose of reducing a particular type of voltage drop, which occurs for the following cause, among various types of voltage drops which may occur when the heating portions are driven. The cause is that, in a case where a one-piece common wire covers all the heating portions, the length of the common wire to each of the heaters differs from one heaters to another depending on the positions of the heating portions. In the case of a configuration disclosed in Japanese Patent Application Laid-open No. 10-44416 (1998), basically, wire resistance is intended to be reduced by making the widths of the common wires as large as possible, for the purpose of reducing voltage drop. On the basis of this, the wire resistances respectively of the electrode wires are designed to be equal to each other by making the widths of the wires different from each other depending on the lengths of the wires to the corresponding units as shown by reference symbols A and B in FIG. 10.
Among inkjet printing apparatuses of these years, however, apparatuses using printing heads with the following configuration have come mainstream. In the case of this configuration, each of such printing heads includes a plurality of ink supply ports in one board, and a plurality of heaters are integrated in a high density in association with each of the ink supply ports, for the purpose of making it possible to record high-definition images each with high image quality at a high speed. For this reason, if the number of simultaneous drives is increased on the basis of the aforementioned prior art for the purpose of realizing the high-speed printing, the connection using the divided wires increases a total of the widths of the wires to a large extent. A resultant drastic enlargement of the size increases costs.
On the other hand, there has been a tendency that printing heads and the like become miniaturized, and this tendency imposes a restriction that the printing heads can not be constructed in a larger size easily. Under such a restriction, it is very difficult to miniaturize boards as long as the conventional connection method using divided wires continues to be used.
With this taken into consideration, the aforementioned divided wires are designed to be used as common wires with low resistance. This makes it possible to check boards from being constructed in a larger size. For the purpose of reducing resistance, it is a general practice to form the common wires in a thicker and wider film. Moreover, for the purpose of electrically connecting a board for an inkjet printing head to the outside, it is a general practice to use electrode pads arranged in an edge of the board for an inkjet printing head.
In addition to the aforementioned common wires, however, wires for individually driving the heaters in accordance with printing data are needed for the board for an inkjet printing head. For this reason, a number of electrode pads have to be arranged in the board for an inkjet printing head. This imposes a limit on areas respectively of the electrode pads in the edge of the board. Consequently, even if the widths of the common wires are wider on the surface of the board, the widths of the common wires have to be equal to or less than those of the electrode pads in the vicinities of the electrode pads for the purpose of connecting the common wires respectively to the electrode pads (see FIG. 11). This increases values of the wire resistances in the edge of the board for an inkjet printing head. Accordingly, this brings about a problem of offsetting the advantage that the common wires are made wider for the purpose of reducing the resistances.